In patients, the anterior cruciate ligament (ACL) is a frequently injured knee structure and its reconstructions after surgery can fail in up to 27% of cases. Losing ACL function can dramatically alter joint kinematics, causing adverse loading on secondary ligamentous restraints in the short to intermediate term and cartilage damage, degenerative joint disease and osteoarthritis in the longer term. Investigators are now beginning to accurately characterize 3-D kinematics in the normal knee. However, critically important 3-D tissue forces that control these kinematics remain impossible to directly measure in patients and can only be inferred by recording ligament surface strains. Ligament forces can conceivably be determined in animal models, but only if: a) joint motions and tissue force transducer outputs are simultaneously recorded in vivo for activities of daily living (ADLs); and b) these instrumented limbs are then calibrated during simulations of these in vivo motions in the laboratory. In this grant, we propose to do just this: to measure in vivo knee motions for multiple ADLs in the normal and injured adult sheep knee and then use robotics to simulate these ADLs in the laboratory in order to determine the 3-D ACL force. These results will provide, in a large and relevant preclinical model, the ability to set design criteria and evaluation benchmarks for both existing and novel ACL treatment strategies such as tissue engineered repairs. To address these needs and build on our preliminary data, we now propose studies in the sheep model to examine activity-related changes in vertical ground reaction forces (VGRFs), knee motions, and total knee and ACL forces before and after controlled medial collateral ligament (MCL) injury and/or medial meniscectomy (MM). Our current studies are designed to answer five important research questions: 1) How does altering gait surface inclination in the normal sheep knee affect these response measures (VGRFs, knee motions, and knee and ACL forces)? How does 2) MCL injury or 3) medial meniscectomy affect these measures? 4) How does combining MCL injury and medial meniscectomy affect these measures? 5) How does combining these two injuries alter these response measures compared to those for both individual injuries? Aim 1: Examine how, compared to level and uphill surfaces, walking a sheep on a downhill gait surface alters normal knee VGRFs, 3-D knee motions, ACL force, and total knee force. Aim 2: Determine how MCL injury or medial meniscectomy influences all response measures for three gait surface inclinations and one speed at 12 weeks post surgery. Aim 3: Determine how combined MCL injury and medial meniscectomy influence our set of response measures for identical gait surface inclinations at 12 weeks post surgery. Contrast these results with those of the individual injuries.